Every case is so different, but working in the child mental health field in the Denver Metro area, I do know that many children remained on their medications. The sunshine and active outdoor life may help, but remember there are still winter days and days indoors and true depression is more related to hormones and SSRI's typically help the most
I would discuss the issue with your doctor or mental health provider, and possibly contact Children's Hospital in Denver just to get a read on their feelings about the issue as well. If he does suffer from a seasonal affective disorder he may benifit, but generalized anxiety, situational anxiety, and depression type symptoms may not be effected tremendously (but who's to say -- trying wouldn't hurt right

). Of course the typical disclaimer of talk with your doctor before making any medication changes
Best of luck to you and your family!